On Tuesday, Mr Fillon, 63, was personally placed under formal investigation over suspicions that he arranged for his wife Penelope to be paid public money for work as his parliamentary assistant which she did not actually carry out.

He is also being investigated over payments to his two children Marie and Charles when he was a senator. Mr Fillon has said his children were paid as lawyers, for specific tasks. But neither was a qualified lawyer at the time.

In all, Mr Fillon is suspected of diverting public funds, complicity in misappropriating funds, receiving the funds and not declaring assets fully.

'Political assassination'

A magistrate had already been investigating the case, but until now the inquiry did not mention Mr Fillon directly.

This is the moment that Francois Fillon feared, but which he knew was probably coming, the BBC's Hugh Schofield in Paris reports.

The embarrassment is acute, he says, because this is the same Mr Fillon, who before the campaign, said it would be inconceivable for someone to remain as a candidate if placed in this legal situation.

The comments are now coming back to haunt him, our correspondent adds.

To explain his decision to maintain his candidacy, Mr Fillon has said he had assumed justice would be fair, but he describes the investigation as a "political assassination".

He also argues that he defeated other centre-right candidates in a primary last November and there is no viable alternative contender.

In a separate development on Tuesday, prosecutors opened an investigation into possible favouritism over the organisation of an event in 2016 by a unit of the economy ministry when it was headed by Mr Macron, AFP news agency reports.

The event in Las Vegas was organised without a public tender. An unnamed aide to Mr Macron denied wrongdoing and told AFP that the investigation would look into Business France, the unit tasked with organising the event.

"It's in no way a story about Macron," he said.

Meanwhile, French media also report that Marine Le Pen is now suspected by the country's tax authorities of undervaluing her share of two properties jointly owned with her father Jean Marie Le Pen.

One of the properties is the family home west of Paris. She denies the allegations and says she will contest the case.

Earlier this month, the European Parliament lifted Ms Le Pen's immunity from prosecution after she tweeted pictures of so-called Islamic State violence.